www/node6: Update 6.12.2 -> 6.12.3

https://nodejs.org/en/blog/release/v6.12.3/

While here, pet portlint:

• move ONLY_FOR_ARCHS
• remove USES_LDCONFIG (node.js doesn't install a .so)

Approved by: robak (mentor)

$ pwd
/usr/ports/www/node6
$ portlint -C
WARN: Makefile: possible direct use of command "python" found. use ${PYTHON_CMD} instead.
WARN: Makefile: possible use of absolute pathname "/etc/make.conf".
WARN: Makefile: "IGNORE" has to appear earlier.
0 fatal errors and 3 warnings found.

http://35.177.216.92/data/latest-per-pkg/node6/6.12.3/